1. The Field of the Invention
The invention relates to wafer materials processing. More particularly, the invention relates to a system and method for making a narrow track-width read sensor for a magnetoresistive head.
2. The Relevant Art
Magnetic head assemblies are typically made of multiple thin film layers which are patterned to form various shaped layers in the head. Some of the layers are plated while other layers are sputter deposited on a wafer substrate. The read head portion of a magnetic head assembly includes multiple layers that are typically sputter deposited. For example, the multiple layers of a read sensor, hard bias and lead layers connected to the sensor and first and second read gap layers below and on top of the sensor are typically sputter deposited. A prior art method of forming shaped sputter deposited layers is to sputter deposit a full film layer of the required material on a wafer substrate, form a patterned resist layer on the layer, ion mill away the exposed portion of the layer and then remove the resist layer leaving the desired shaped layer that was protected therebelow.
The aforementioned method of shaping sputter deposited layers has been generally superseded by a bilayer lift-off mask scheme which is fully explained in commonly assigned U.S. Pat. No. 5,018,037, which is incorporated by reference herein. The bilayer lift-off mask has a T-shape as seen in cross section wherein the vertical portion of the T is short and wide but less wide than the horizontal top portion of the T. The top portion of the T is generally a patterned resist layer and the bottom vertical portion of the T is a release layer. The configuration provides first and second undercuts as seen in cross section wherein each undercut has a height and a length below the resist portion. In the aforementioned patent the bilayer lift-off mask is employed for the purpose of making contiguous junctions of the first and second lead layers with first and second side edges respectively of the read sensor. Multiple read sensor layers are sputter deposited in full film on the wafer substrate followed by formation of the bilayer lift-off mask covering a read sensor site. Ion milling is then employed to remove all of the read sensor material except that below the mask. Full films of hard bias and lead layer materials are then sputter deposited to cover the top of the lift-off mask and an area surrounding the lift-off mask. The heights of the undercuts are generally greater than the thickness of the hard bias and lead layers. This is so a resist stripper can reach the release layer. The stripper is then introduced to dissolve the release layer, causing the bilayer lift-off mask and the hard bias and lead materials deposited thereon to be released from the wafer substrate, resulting in the aforementioned contiguous junctions between the first and second lead layers and the first and second side edges respectively of the read sensor.
The method of the aforementioned patent is currently considered not to be precise enough to implement contiguous junctions between the read sensor and the lead layers. Prior to that patent the lead layers overlapped the top of the read sensor and were constructed with a second resist mask. Since patterning of resist masks is not precise enough to align a second mask with side walls created by a first mask, the overlapping scheme was necessary. Unfortunately, this scheme caused the hard bias and lead layers to form a high profile on top of the read sensor which was replicated through subsequent layers into a write gap of the write head causing a curvature of the write gap. Write gap curvature degrades the performance of the head, since the write head writes curved magnetic bits of information into the rotating disk while the read head reads the magnetic bits of information straight across. This causes a loss of signal at the outside lateral edges of the track width of the read head.
Accordingly, the bilayer lift-off mask scheme has significantly improved the fabrication of read heads by forming contiguous junctions between the lead layers and the read sensor. Fewer processing steps are required and the profile of the lead and hard bias layers above the read sensor has been reduced. Unfortunately, present bilayer lift-off masks are limited to fabrication of read heads with an insufficently narrow track width. The narrower the track width, the greater the number of tracks per inch (TPI) that can be read by the read head from a rotating magnetic disk. Accordingly, the greater the tracks per inch, the greater the storage capacity of a disk drive employing such a read head. Process control of the undercut has been a significant limitation in the creation of narrower track widths. If the undercut is too deep, the underlying release material will be too narrow and mechanically unstable and can cause the bilayer lift-off mask to be separated from the substrate or to topple over during resist development or subsequent processing steps of ion milling and sputter deposition. If the undercut is too shallow, sputtered material can be deposited on the exposed sides of the release layer. This material can prevent the stripping solvent from completely dissolving the underlayer during lift-off and can allow sputtered material to be left behind. This material left behind can be in the form of protrusions too high to be covered by the thin gap insulator layer, resulting in interlevel shorting paths between the sensor and the overlying shield. These problems can be attributed, at least in part, to the wet resist development step that serves to remove exposed (positive-tone) resist, remove the portion of the release layer underneath the exposed resist, and undercut the portion of the release layer underneath the exposed resist.
At the size scale involved in fabrication of the read head, surface tension forces become significant and potentially destructive. As a fluid dries, surface tension exerts forces on the remaining portions of the resist and release layers. Attempts to produce a narrower read sensor require the production of an even narrower release layer pedestal that results in a structure that may be easily deflected or even toppled in response to surface tension as the developer dries, causing yield and quality control problems. Mechanical agitation, commonly used to in the developing process, also becomes more problematic with narrower pedestals. In addition, variations in surface tension and wetability can lead to variations in undercut lengths in the wet process that exceed allowable tolerances. Accordingly, there is a need for a process of making a bilayer lift-off mask that produces highly controlled undercuts with an appropriately narrow width. Furthermore, there is a need for such a process that is capable of consistently producing bilayer lift-off masks that are not exposed to fluid forces. Yet further, there is a strong-felt need for such a process that is comparatively inexpensive to carry out with commercially available wafer material processing equipment.